We propose to study aspects of the responses to sound of cells in the cochlear nucleus (CN) and inferior colliculus (IC) of cats. Extracellular recording will be used to characterize response properties of single units tomonaural and binaural stimuli. The aim is to develop a classification scheme for response properties in these nuclei in unanesthetized intact preparations. In the case of the CN, we expect to verify or refine the classifications already developed for anesthetized or decrerebrate preparations. In the IC, we will apply a comprehensive approach based on response maps and binaural connectivity properties to develop a new classification scheme for response types. These classification schemes are an essential basis for more detailed study of these nuclei and for the study of information processing in the auditory system. A second aim is to study the responses of units in the CN to a variety of stimuli in the presence of background sound. Auditory-nerve fibers have been shown to be incapable of true adaptation, in the sense that their dynamic ranges are fixed; high level background noise produces a saturated or nearly saturated response in all fibers. Preliminary evidence indicates that at least some neurons in the cochlear nucleus are capable of shifting their operating point to compensate for background stimuli and thereby can maintain dynamic responsiveness to stimuli presented against the background. We will determine which response types in which parts of the CN are capable of adaptation and will study some properties of the adaptation. This work will contribute to our eventual understanding of the mechanisms by which the auditory system maintains its very wide dynamic range.